Insulative, non-wetting mold coverings have been, and continue to be, part of the strategy to eliminate the problem of uneven heat transfer and its attendant bad effect on the metallurgy of the cast product of moving-mold continuous casting machines. These non-wetting coverings include permanent pre-coverings or base coverings (hereinafter called "basings"). These are described in U.S. Pat. No. 4,588,021 of Bergeron et al. Also, there are the more or less temporary top deposits or top dressings or temporary insulative deposits or toppings or mold-release agents, which are applied on top of a basing. All prior-art top or temporary insulative deposits known to us wear and compact and flatten unevenly and thus soon require replenishment or replacement. Manual replenishment of the unevenly worn or flattened spots does not in practice result in re-establishing a top deposit that affords uniform heat transfer. Nor has it been feasible to strip and reapply the prior-art insulative toppings, which usually comprise a binder.
Most of the prior-art top deposits were applied wet. Thus, residues of liquid resulting from such wet applications would sometimes flash into gas and cause porosity or other problems in the cast product. In the casting of copper bar or copper anodes in belt-type machines, synthetic oils upon otherwise bare metallic casting belts have been customary, sometimes resulting in similar porosity problems. None of the prior art known to us can achieve the unique results disclosed herein.
There is a prior-art method for continuous casting of metal in a belt-type machine, the method comprising an operation of feeding molten metal into a mold region defined by two flexible, continuously moving, water-cooled casting belts having workfaces (U.S. Pat. No. 3,795,269, 164/73, of Leconte et al., issued 5 Mar. 1974). A two-layer dressing is applied to each casting surface. The first layer is a basing dressing which includes a heat-insulating coating fixedly adhered to the workface of the casting belt. The second layer is a removable parting layer of dry powder particles, deposited over said basing layer. As elements of the casting surface move successively out of and into engagement with the metal being cast during each cycle of operation, the casting surface is cleaned to remove the previously applied parting layer of powder particles, and a fresh parting layer of powder particles is newly applied. There are two assemblies for applying a temporary insulative coating respectively to two casting belts.
Each assembly for applying the parting layer of powder particles is made as a hopper from which a layer of dry powder particles is scattered out, continuously covering the casting belt. This temporary parting layer is later removed by means of rotating steel brushes (U.S. Pat. No. 3,795,269).
Our opinion as to the patent of Leconte et al. is that it does not describe the invention in terms that would enable one to carry it out. Specifically, insulative parting-layer powders must be applied in very thin coatings, lest the metallic product cast against them be contaminated or the product surfaces damaged. Moreover, the required thin coatings of powder must be applied in a quite uniform thickness, lest the rate of heat transfer in the freezing process become nonuniform in different areas of the casting belts, a condition that results in bad metallurgical properties in the cast product. Leconte et al. have not specified how they will apply such thin, uniform powder coatings. They mention only "a hopper distribution system" (column 5, lines 37-40). Anyone who has handled talc or other powder particles in bulk knows that this Indefinite disclosure will not suffice as a description of what must be done to achieve a suitable thin, uniform coating. The teaching of Leconte et al. as disclosed is imperfect. Further art is required to apply the powder in a suitable thin, uniform coating required in the art of continuous casting of metals upon moving cooling surfaces, especially upon flexible casting belts.
The task thus set for the present invention is to provide the method and the apparatus for increasing the service life of a mold surface while at the same time increasing the uniformity of heat transfer during successive contacts between the workface of a mold surface and the molten metal being continuously cast.